In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used. One means believed effective for further reducing the feature size is to reduce the wavelength of exposure light. For the mass production process of 64 M-bit dynamic random access memory (DRAM, processing feature size 0.25 μm or less), the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 256 M and 1 G or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source is required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the postponement of F2 lithography and the early introduction of ArF immersion lithography were advocated (see Proc. SPIE Vol. 4690 xxix).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE Vol. 5040, p 724).
Several problems associated with the presence of water on resist were pointed out. For example, projection lens contamination and pattern profile changes occur because the acid once generated from a photoacid generator and the amine compound added to the resist as a quencher can be dissolved in water. Inversely, swelling and circular defects known as water marks occur because water can penetrate into the resist film. For overcoming these problems, it was proposed to provide a protective coating between the resist and water (see the 2nd Immersion Workshop, Jul. 11, 2003, Resist and Cover Material Investigation for Immersion Lithography); and to prevent resist materials from dissolution in water or water penetration by controlling the water repellency of resist materials, typically photoacid generators (PAG) or base resins (see J. Photopolymer Sci. and Technol., Vol. 18, No. 5, p 603 (2005)).